I think that future historians will love the Internet because it has brought the written word back into day-to-day human affairs.
Up until the mid-20th Century, letters were the primary means of non-face-to-face communications. Letter writing was so important to social and political dialog of the 18th Century that the American founders created the flat-rate postal service within the U.S. Constitution itself. Over the course of a lifetime, a politically, socially or economically active individual would write thousands of letters, often several a day. It was common practice to save such communications. For historians, these are a valuable insight into the thinking of the people corresponding. The tradition of leaving one’s “papers” to an institution is largely a matter of archiving one’s lifetime correspondence.
The rise of the telegraph and then the telephone muted the historical record. People rarely saved telegrams and almost nobody recorded telephone calls. From WWII up until the mid-’90s, a lot of historically important dialog disappeared into the ether. The history of the era is largely an oral one, collected post hoc from survivors. Such oral histories, heavily shaded by hindsight and contemporary sensibilities, give a much different view of events than written communications done as part of actual events. We especially miss the evolution of ideas over time.
The rise of the Internet has brought written communication back into the day-to-day world and given us a means of capturing the thinking of people for the historical record. Computer historians have already used early Internet archives to rewrite the early history of the Internet itself (which was what most of the early dialog on the Internet was actually about). Future scholars of the “9/11 era” and beyond will lean heavily on Internet sources to understand the thinking of people in the early-21st Century.
My advice? Save everything you write. These days it is cheap and easy to do so. You never know what future historians will want to know about your life. Even trivial emails or IMs about what groceries to get might become part of somebody’s doctoral thesis someday.